Methods and devices for cell discovery

ABSTRACT

This disclosure sets forth methods and devices for communication between mobile devices and base stations with active and dormant states. In an embodiment, a base station transmits system information during an active state of the base station with at least one system-information message. The at least one system-information message includes a SystemInformationBlockType1 (“SIB1”) message with a first update-indicator field. The base station selects an update value that indicates whether the system information has changed since a previous transmission of a previous SIB1 message. The base station transmits at least one dormant-state message during a dormant state of the base station with the selected update value in a second update-indicator field of the at least one dormant-state message.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplications 61/807,832, filed Apr. 3, 2013, and 61/807,836, filed Apr.3, 2013, the contents of which are incorporated herein by reference intheir entireties. This application is related to U.S. patent applicationSer. No. 61/807,836, filed on an even date herewith.

TECHNICAL FIELD

The present disclosure is related generally to wireless networkcommunications and, more particularly, to communication between mobiledevices and base stations with active and dormant states.

BACKGROUND

Mobile devices for wireless networks scan for signals and channels inorder to synchronize with the wireless network. Where the wirelessnetwork is a Long-Term Evolution (“LTE”) network, an evolved Node B(“eNB”) is a base station that broadcasts primary synchronizationsignals (“PSS”) and secondary synchronization signals (“SSS”) ascell-identification signals in each system frame. The eNB alsoperiodically transmits other general signals that are not specific to amobile device, such as common reference signals, and networkconfiguration data, such as a master information block and systeminformation blocks. The eNB consumes power to transmit the generalsignals and network configuration data even when there are no mobiledevices with an active connection. In some cases, the eNB is configuredfor a reduced activity state or dormant state. At least while mobiledevices are connected to the eNB, the eNB must provide a notification tothe mobile devices when the network configuration data have changed.

The eNB must also periodically transmit a cell-identification signal sothat other mobile devices can detect the eNB, for example, for ahandover between eNBs or when a user turns on his mobile device.Cell-identification signals such as the PSS and SSS are susceptible to“pilot pollution” and “pilot collision” when cells are very close toeach other. For example, a wireless network operator can place severalsmall cells in an area where he wishes to offload data from a macrocell. In this case, the cell-identification signals from each small cellcan cause interference with other small cells which are in closeproximity.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

While the appended claims set forth the features of the presenttechniques with particularity, these techniques, together with theirobjects and advantages, may be best understood from the followingdetailed description taken in conjunction with the accompanying drawingsof which:

FIG. 1 is an overview of a representative communication system in whichthe methods of this disclosure may be practiced;

FIG. 2 is a block diagram of a base station of the system of FIG. 1,according to an embodiment;

FIG. 3 is a block diagram of a mobile device of the system of FIG. 1,according to an embodiment;

FIG. 4 is a flowchart of a method carried out by the base station ofFIG. 1, according to an embodiment;

FIG. 5 is a flowchart of a method carried out by the mobile device ofFIG. 1, according to an embodiment;

FIG. 6 is a flowchart of another method carried out by the mobile deviceof FIG. 1, according to an embodiment;

FIG. 7 is a flowchart of yet another method carried out by the mobiledevice of FIG. 1, according to an embodiment;

FIG. 8 is a flowchart of a method carried out by the base station ofFIG. 1, according to an embodiment;

FIG. 9 is a flowchart of a method carried out by the mobile device ofFIG. 1, according to an embodiment;

FIG. 10 is a flowchart of another method carried out by the mobiledevice of FIG. 1, according to an embodiment; and

FIG. 11 is a diagram illustrating radio frames transmitted by a basestation of the system of FIG. 1, according to an embodiment.

DETAILED DESCRIPTION

Turning to the drawings wherein like reference numerals refer to likeelements, techniques of the present disclosure are illustrated as beingimplemented in a suitable environment. The following description isbased on embodiments of the claims and should not be taken as limitingthe claims with regard to alternative embodiments that are notexplicitly described herein.

The various embodiments described herein allow a base station or cell(e.g., a small cell) to indicate changes in system information during adormant state of the base station or cell. This technique helps toreduce signaling overhead between a cell and mobile devices thatdiscover or are served by the cell. In other embodiments, the basestation transmits a cell-discovery signal that allows a mobile device todetermine whether the base station is in an active or dormant state.This technique allows a mobile device to select radio resources based onthe state of the base station. In still other embodiments, the basestation selects a cell-discovery signal that indicates a small-cellidentifier selected from a set of allowable values that is larger than anumber of available physical-cell identifiers. This technique reducesinterference between discovery signals of adjacent small cells.

According to an embodiment of the disclosure, a base station transmitssystem information with at least one system-information message (“SImessage”) during an active state of the base station. The SI messageincludes a SystemInformationBlockType1 (“SIB1”) message with a firstupdate-indicator field. The base station selects an update value thatindicates whether the system information has changed since a previoustransmission of a previous SIB1 message. The base station transmits,during a dormant state of the base station, at least one dormant-statemessage with the selected update value in a second update-indicatorfield of the at least one dormant-state message.

Turning to FIG. 1, a block diagram 100 illustrates base stations 110,120, 130, and 140 and a mobile device 150. The base stations 110, 120,130, and 140 form at least part of a wireless network 160. In oneembodiment, the wireless network 160 is a cellular (e.g., LTE) network.In the embodiment shown, the base station 110 controls a macro cell 111and the base stations 120, 130, and 140 control small cells 121, 131,and 141, respectively. The small cells 121, 131, and 141 are associatedwith the macro cell 111, for example, the base stations 120, 130, and140 are controlled by the base station 110. Examples of small cellsinclude femtocells, picocells, and microcells. The mobile device 150communicates with the wireless network 160 via the base stations 110,120, 130, or 140. Possible implementations of the mobile device 150include a mobile phone (e.g., smartphone), a tablet computer, a laptop,or other accessory or computing device.

Turning to FIG. 2, a block diagram 200 illustrates an embodiment of abase station such as the base stations 110, 120, 130, and 140 of FIG. 1.Possible implementations of the base station include an EvolvedUniversal Terrestrial Radio Access base station, an eNB, a transmissionpoint, a Remote Radio Head, a home eNB, or a femtocell. In one example,the base station is an eNB that controls a macrocell of the wirelessnetwork 160. In another example, the base station controls a small cellof the wireless network 160. In yet another example, the base stationcontrols a macro cell and one or more small cells of the wirelessnetwork 160. The base station in other examples includes multiplenetwork entities. For example, the base station can in fact be two ormore base stations operating in conjunction with one another to operateas a single base station or network entity. The base station in oneexample is a portion of another network entity.

The base station includes a transceiver 202, which is configured totransmit data to and receive data from other devices such as the mobiledevice 150. The base station also transmits or broadcasts signals ordata within the corresponding cell. For example, the base station 110transmits signals within the macro cell 111, and the base station 120transmits signals within the small cell 121. The base station alsoincludes at least one memory 204 and a processor 206 that executesprograms stored in the memory 204. The processor 206 writes data to andreads data from the memory 204. During operation, the transceiver 202receives data from the processor 206 and transmits Radio Frequency(“RF”) signals representing the data. Similarly, the transceiver 202receives RF signals, converts the RF signals into appropriatelyformatted data, and provides the data to the processor 206. Theprocessor 206 retrieves instructions from the memory 204 and, based onthose instructions, provides outgoing data to, or receives incoming datafrom, the transceiver 202.

The base station transmits signals and data for synchronization ofmobile devices with the wireless network 160. In an embodiment, thewireless network 160 is an LTE network or LTE-Advanced network. In atypical LTE network, the base station transmits cell-identificationsignals in each system frame, such as PSS and SSS. The base station alsoperiodically transmits other general signals (i.e., signals not specificto a mobile device), such as common reference signals, and data, such asa master information block transmitted over a physical broadcast channel(“PBCH”), SIBs transmitted over a physical downlink shared channel(“PDSCH”), and others as will be apparent to those skilled in the art. Amobile device discovers a cell and determines the physical-cellidentifier for the cell based on the PSS and SSS. The cell consumes morepower when transmitting PSS and SSS during an active state. The PSS andSSS are also susceptible to “pilot pollution” and “pilot collision” whencells are very close to each other (e.g., for the small cells 121, 131,141).

The base station operates in multiple states, such as one or more of anactive state, dormant state, or semi-dormant state. In general, in thedormant state, the periodicity of periodic non-mobile device-specifictransmissions (e.g., synchronization signals, transmissions related tosystem information) from the base station is longer (e.g., “1 ms every100 ms” or “5 ms every 1 second”) when compared to the periodicity ofsuch transmissions in the active state (e.g., “1 ms every 5 ms” or“multiple symbols in every 1 ms subframe”). Enabling a base station tooperate in a dormant state not only helps in reducing energy consumptionof the base station but also helps in reducing overall networkinterference.

In some embodiments disclosed herein, the base station uses acell-discovery signal or small cell-discovery signal (“SCDS”), inaddition to the PSS and SSS, for synchronization of mobile devices withthe wireless network 160. The SCDS in one example allows a mobile deviceto determine a cell identifier (e.g., a small-cell identifier), adownlink cyclic prefix length, half-frame timing, and slot index. Wherethe PSS, SSS, and SCDS are used within a same frame, the base stationtransmits the PSS, SSS, and SCDS using non-overlapping radio resources(e.g., resource elements or resource blocks (“RBs”)). The base stationin one example selects the radio resources for the SCDS such that theSCDS is detectable by a mobile device within a single measurement-gapinstance (i.e., 6 ms for LTE). In a further example, the base stationtransmits the SCDS using radio resources that do not overlap with radioresources for a physical control format indicator channel (“PCFICH”), aphysical downlink control channel (“PDCCH”), or physical hybridautomatic repeat request indicator channel (“PHICH”).

The base station described herein transmits the SCDS and thus provides asynchronization signal that can be transmitted with a longer periodicity(e.g., once every 100 ms or multiple seconds) than the periodicity usedfor PSS and SSS transmission (once every 5 ms). Also, in order to reduceinter-frequency measurement burdens for mobile devices, it is desirableto have an SCDS structure that is detectible by the mobile device in asfew subframes as possible. In specifications for LTE Releases 8, 9, 10,and 11 (3rd Generation Partnership Project; 3gpp.org), eachmeasurement-gap instance allows a mobile device to performinter-frequency measurements within 6 ms. Therefore it is desirable ifthe mobile device can detect the SCDS of multiple cells within 6 ms(i.e., within a single measurement-gap instance). It is also desirablethat the base station transmits the SCDS in both the dormant and theactive states. In this case, the resource-element positions used forSCDS can be selected such that they do not overlap the resource-elementpositions used for PSS, SSS or PBCH. Further, only subframes 0, 1, 5,and 6 are guaranteed to have downlink transmissions for time divisionduplexing. Thus, it is desirable to limit SCDS transmission to subframes0, 5 and the first two symbols of subframes 1 and 6. SCDS transmissionscan overlap with resource elements used for PCFICH, PDCCH, or PHICH, butthis would imply that legacy mobile devices cannot receive controlsignaling in subframes with SCDS transmission. This is an undesirablescheduling restriction. Thus, it also desirable to have theresource-element positions of SCDS not overlap with PDCCH, PCFICH, orPHICH resource-element positions.

Based on the above factors, an SCDS structure with a 6 resource-blockbandwidth that can be detected within 1 subframe is likely to have avery short range (i.e., it can only be operated in a high signal tointerference plus noise regime). On the other hand, if the SCDS isallowed to span multiple subframes (e.g., 6 subframes) to increaserobustness, then the time needed to detect the SCDS approaches that ofPSS and SSS, thereby reducing the need for a new structure. Consideringthis, at least for 1.4 MHz carrier bandwidth case, supporting a separateSCDS signal in addition to PSS and SSS is less beneficial. If a dormantstate is to be enabled on a base station for this narrow bandwidth case,then the base station can transmit PSS and SSS in bursts with a longerperiodicity. For example, PSS and SSS can be transmitted in two or threeradio frames (i.e., 4 or 6 occasions of PSS and SSS) every few seconds.Therefore, the SCDS can be substituted for PSS and SSS transmitted inbursts with a longer periodicity. Assuming a minimum bandwidth of 5 MHz(e.g., 25 RBs) for SCDS opens up possibilities for a compact and robustdesign. SCDS design can be similar to Positioning Reference Signal(“PRS”) design introduced in LTE Release 9. In this case, two bandwidthscan be initially used, with more bandwidths added if needed. An exampleusing bandwidths of 1.4 MHz and 5 MHz is described with respect to FIG.10.

The base station periodically transmits a dormant-state message over asmall-cell-discovery channel (“SCDCH”) or dormant physical broadcastchannel (“D-PBCH”) during the dormant state, according to anotherembodiment. The dormant-state message allows the base station to conveyinformation which cannot be implicitly embedded in an SCDS transmissionwhen the cell is in the dormant state. The base station transmits theSCDCH with a greater periodicity than the PSS and SSS. In someembodiments, the base station transmits the SCDCH with a greaterperiodicity than the SCDS. In one example, the base station transmitsthe SCDS and the SCDCH as the only general or broadcast transmissions(e.g., not specific to a mobile device) during the dormant state.

When the base station is in the active state, it transmits all thesynchronization signals and broadcast channels required to supportmobile devices compliant with specifications for LTE Releases 8, 9, 10,and 11 (e.g., legacy mobile devices). Therefore, in the active state, atleast the following periodic generic (e.g., non-mobile device-specific)transmissions are made by the base station: PSS in slot 0 and 10 ofevery radio frame, SSS in slot 0 and 10 of every radio frame, physicalbroadcast channel carrying MasterinformationBlock (“MIB”) in slot 1 ofevery radio frame in the active state, PDSCH carrying SIB1 informationin every alternate radio frame (i.e., radio frames satisfying SystemFrame Number (“SFN”) mod 2=0) and associated PDCCH to indicate the PDSCHRBs, PDSCH carrying other SIBs in a plurality of radio frames conformantwith the system information scheduling mechanisms in LTE Releases 8, 9,10, and 11 and associated PDCCH to indicate the PDSCH RBs, and commonreference signals (“CRS”) in every slot of every radio frame except forthe second slot in a Multicast Broadcast Single Frequency Network(“MBSFN”) subframe (MBSFN subframe information typically signaled inSIB2). Given the above list of signals and channels, the base station,in the active state, has at least one transmission in every slot, i.e.,a periodicity of once every 0.5 ms.

In addition to the transmissions in the above list, the base station canalso transmit one or more of the cell-discovery signal or cell-discoverychannel when in the active state. If the cell-discovery signal orcell-discovery channel has a structure that is detectible in fewer slotsthan the slots required for detecting the PSS and SSS, then thetransmission of the cell-discovery signal or cell-discovery channel inthe active state can help in reducing the measurement burden of mobiledevices making inter-frequency measurements on the transmitting cell.For example, a first cell can transmit a discovery signal and discoverychannel on a carrier with center frequency f₁ and with a periodicity ofonce every 150 ms. A mobile device connected to a second cell operatingon a carrier with center frequency f₂ can attempt to detect the firstcell by either attempting to detect the PSS and SSS on the first cell orthe discovery signal on the first cell. Using techniques describedherein, the discovery signal transmission can be made more robust thanPSS and SSS transmissions so that a mobile device is able to detect thefirst cell in “one shot” if it uses the cell-discovery signal (i.e., byreceiving only one instance of the cell-discovery signal) instead of thePSS and SSS, which requires a longer detection time (e.g., receiving 4or 5 instances of PSS and SSS) in certain conditions.

The base station transmits the SCDS in all slots or a subset of slotswithin a radio frame based on a first periodicity. The first periodicityis greater than a second periodicity of the PSS and SSS or other signalstypically transmitted in the active state. In LTE, a radio framecontains 10 subframes, where a subframe is 1 ms duration and includestwo slots, each slot of 0.5 ms duration. Only subframes 0, 1, 5, and 6are guaranteed to have downlink transmissions for a time division duplexconfiguration, thus, the base station in one example transmits the SCDSusing subframes 0, 5 and the first two symbols of subframes 1 and 6.Examples of the first transmission duration and first periodicityinclude 1 millisecond every 100 milliseconds, 5 millisecond every 1second, or every 15^(th) system frame, while examples for the secondtransmission duration and second periodicity include 1 millisecond every5 milliseconds or multiple symbols in every 1 millisecond subframe.Other values for the first or second periodicities will be apparent tothose skilled in the art. The greater periodicity of the SCDS allows forreduced power consumption by the base station and the mobile device forsynchronization and also reduces overall network interference. In somecases, the base station uses the PSS and SSS during an active state anduses the SCDS during a dormant state.

Turning to FIG. 11, a base station (or a cell) can transmit one or moreof a pilot signal, reference signal, or synchronization signal (usuallyreferred to as a cell-discovery signal) in radio frames when operatingin a dormant state. In one implementation, a base station canperiodically transmit the cell-discovery signals. The base station canalso periodically transmit a physical broadcast channel (usuallyreferred to as a cell-discovery channel) that is associated with thecell-discovery signal with a same or longer periodicity when in thedormant state. When the base station is in an active state, the basestation can transmit additional synchronization signals and broadcastchannels with a shorter periodicity when compared to the dormant state.

FIG. 11 illustrates multiple radio frames, some transmitted when thecell is in active state, and some transmitted when the cell is indormant state. In LTE, each radio frame has 10 ms duration and consistsof 20 slots numbered from 0 to 19. Each slot is 0.5 ms. Consecutiveslots are usually referred to as subframes (e.g., slot 0, 1 is onesubframe, slots 2, 3 another subframe etc.). The radio frames areusually indexed with an SFN. When the cell is in the dormant state asshown in FIG. 11, it transmits the discovery signal or discovery channelin every 15^(th) radio frame (i.e., radio frames satisfying SFN mod15=0). The discovery signal or discovery channel can be present in allslots or a subset of slots within that radio frame (e.g., slots 0, 1, 9,10). The discovery signal or discovery channel transmissions can be theonly periodic non-mobile device-specific transmissions (e.g., generictransmissions) made by the cell in the dormant state.

In other embodiments, the base station transmits the SCDS in both theactive state and the dormant state. The active and dormant-state SCDSsin one example are transmitted with different periodicities. In thiscase, the base station transmits the SCDS at a shorter periodicity(i.e., more frequently) (e.g., 5 or 10 millisecond intervals) in theactive state than in the dormant state (e.g., 100 ms, 200 ms, 1 second,or more). This technique allows faster discovery and handover to activecells and provides interruption durations similar to techniques providedby LTE Release 8 for a handover from a macro cell to an active smallcell.

In embodiments where the SCDS has a structure that is detectible infewer slots than the slots typically required for detecting the PSS andSSS, the transmission of the SCDS in the active state allows a reductionin a measurement burden on mobile devices making inter-frequencymeasurements on the cell transmitting the discovery signal. The basestation optionally omits the PSS and SSS when transmitting the SCDS inboth the active and dormant states and thus the SCDS is used instead ofthe PSS and SSS. In one example, the base station transmits the SCDS asthe only general signal or data transmission during the dormant state.

The SCDCH in one example has a same bandwidth as the SCDS. The bandwidthof the discovery channel or SCDS may not be equal to the transmissionbandwidth configuration of a carrier. Instead they can be transmittedwith a fixed bandwidth (or a fixed set of blindly detectable bandwidths)that is known a priori or predetermined to the mobile devices. In LTEReleases 8, 9, 10, and 11, PSS, SSS, and PBCH are transmitted with afixed bandwidth of 6 RBs. In one example, the bandwidth of the SCDCH andSCDS signals is fixed at 25 RBs. Alternately, the mobile devices may bemade to blindly detect the bandwidth of these signals from a set ofallowed bandwidths (e.g., 6 RBs and 25 RBs). In a further example, thebase station transmits the SCDCH and the SCDS in the same or adjacentsubframes. This allows the mobile device to use channel estimatesobtained for detection of the SCDS for demodulation of the SCDCH.Alternatively, a predefined pattern of reference signals (e.g., thepattern used for demodulation reference signals or CRS in LTE Release 11specifications) can also be transmitted along with D-PBCH to assist itsdemodulation. This reference signal pattern can also be a function of anidentifier determined during SCDS detection. The identifier may be asmall-cell identifier (“SCID”) or an extended physical-cell identifier(“E-PCID”).

The SCDS and SCDCH in one example have a fixed bandwidth or a set offixed bandwidths (e.g., six RBs or 25 RBs) that are known a priori tothe mobile device, thus enabling blind detection by the mobile device.An RB in a subframe includes 12 subcarriers in frequency and the symbolswithin a slot (e.g., 7 symbols in a slot when normal cyclic prefixduration is used) in time where the subcarrier spacing is 15 kHz. In oneexample, the bandwidth of the SCDS and SCDCH are fixed to 25 RBs toallow for one-shot detection of the SCDS.

In other embodiments, the base station transmits other dormanttransmissions (e.g., signals or channels) in addition to the SCDS andthe SCDCH during the dormant state (e.g., a “semi-dormant” state). Thesedormant transmissions can include a reduced cell reference signaltransmission (i.e., transmission of a pilot sequence in the fifthsubframe of every radio frame on resource elements corresponding to aCRS antenna port), new broadcast channel transmissions associated withdemodulation reference signals instead of CRS (i.e., the demodulationreference signals are used to demodulate the broadcast channeltransmissions), or other new control channel transmissions such as acommon search space for an enhanced physical downlink control channel.Such transmissions can be used by advanced mobile devices (e.g., mobiledevices supporting future releases of LTE specifications such as LTERelease 12) for connecting to and communicating with the cell even whenit is in dormant state. The energy spent by the cell in the dormantstate of this implementation is higher than the energy spent in thedormant state described above (e.g., a dormant state with only the SCDSand SCDCH). However, when compared to the energy spent in the activestate (e.g., where CRS is transmitted in every slot), the overall energyspent is still lower.

In some embodiments, in the dormant or semi-dormant state, the basestation supports some or all of the procedures required to serviceidle-mode mobile devices. In one implementation, the mobile device candetermine various parameters and information from receipt of thedormant-state message.

In some embodiments, if the mobile device has not previously receivedsystem information from a cell, and if it detects the cell while thecell is in the dormant state, it then initiates a cell wake-up procedureto transition the cell from the dormant state to the active state. Themobile device then downloads the system information (e.g., MIB and SIB1through SIB11 as described in LTE specifications) from the cell andstores the value tag (e.g., an integer with range 0 to 31) associatedwith the downloaded system information. The mobile device determines thepaging occasions and paging frames corresponding to its mobile device IDfrom the downloaded system information. The mobile device then monitorsPDCCH or Enhanced PDCCH (“EPDCCH”) for downlink control information withcyclic redundancy check scrambled by a paging radio network temporaryidentity in the paging occasions. The mobile device also continuesmonitoring SCDS and D-PBCH for any change in system information or cellstate in embodiments where the SCDS and D-PBCH are transmitted in theactive state. The mobile device determines whether the systeminformation has changed by comparing the value tag it has stored withthe value tag transmitted in the D-PBCH. In this embodiment, if the cellmoves to the dormant state, then the mobile device does not re-initiatethe wake up procedure to move the cell to active state. Instead, themobile device continues monitoring SCDS and D-PBCH as long as the systeminformation of the cell is unchanged (e.g., as long as the value tagreceived in D-PBCH matches the value tag stored when the mobile devicepreviously received SIB1).

This technique allows a mobile device to leave and re-enter the coveragearea of a base station in an efficient manner. The mobile devicedownloads and store system information from the base station. The mobiledevice then leaves and later re-enters the coverage area of the basestation. If the base station is in the dormant state when the mobiledevice re-enters and the value tag of the system information associatedwith the cell transmitted in the D-PBCH indicates that the systeminformation has not changed, then the mobile device does not wake up thebase station to again download system information or check its validity.Instead, the mobile device uses the value tag transmitted in the dormantstate to determine the validity of the previously stored SI.

In some embodiments, while the cell is in the dormant state (from theperspective of the mobile device), and the mobile device is camped onthe cell in Radio Resource Control (“RRC”) idle mode, the mobile devicecan follow the same procedures as specified in LTE specifications inresponse to the following events: (1) The mobile device detects thatsystem information has changed (either from value tag on D-PBCH or via apaging message indicating the change); (2) The mobile device detects apaging message in its paging occasion; or (3) The mobile device hasuplink data to transmit and has to send a connection request. The cell,however, has to switch from dormant to active state whenever there is asystem information change, or whenever it receives a page message for amobile device which belongs to the same tracking area as the cell, orwhenever it receives a wake up signal or Random Access Channel (“RACH”)from a mobile device camped on the cell.

In some embodiments, the base stations operates in a dormant orsemi-dormant state even when idle-mode mobile devices are camped on thecell. The base station may support some or all of the proceduresrequired to service idle-mode mobile devices. In some embodiments, onlySCDS and D-PBCH (with a small payload containing the value tag) aretransmitted with a longer periodicity (e.g., once every 100 ms or onceevery 1 second) by the base station. In some embodiments, the mobiledevice can wake up more often compared to idle modes in LTE Releases 8,9, 10, or 11, as it has to not only wake up for monitoring its pagingoccasions but also wake up for monitoring SCDS and D-PBCH. The effect ofadditional “wake-ups” can be reduced by making the mobile device monitoran alternative paging occasion that is adjacent (or closer in timedomain) to the SCDS and D-PBCH transmission when the cell is in thedormant state. For example, the alternative paging occasion can bepresent in the same subframe where SCDS or D-PBCH are monitored by themobile device. Alternatively, a subframe offset, which may be relativeto the subframe where SCDS or D-PBCH is monitored by the mobile device,for the alternate paging occasion (different offsets can be applicableto different mobile device IDs) can be signaled in the D-PBCH. Thus, ifthe mobile device determines that the cell is in the dormant state, thenit monitors a first control channel for a paging indication in a firstpaging occasion or a first time/frequency location (the location isdetermined based on SCDS position or a subframe offset signaled inD-PBCH). If the mobile device determines that the cell is in the activestate, then it monitors a second control channel for paging indicationin a second paging occasion occasion or a second time/frequency location(the location is determined from system information received in MIB andSIBs when the cell is in the active state).

A similar approach can be used to modify the RACH transmissions from themobile device to reduce the eNB RACH monitoring instances in dormantstate. If the mobile device determines that the cell is in the dormantstate, then it transmits RACH in a first set of locations (the locationsare determined based on SCDS position or a subframe offset signaled inD-PBCH; the offset may be relative to the subframe where the SCDS orD-PBCH is monitored by the mobile device), and if the mobile devicedetermines that the cell is in active state, then it transmits RACH in asecond set of locations (the locations determined from SI received inMIB and SIBs when the cell is in active state).

In some embodiments, a base station supports an active state asdescribed above, a semi-dormant state with the SCDS and SCDCH and otherdormant transmissions described above, and a dormant state with only theSCDS and SCDCH.

The base station in one example supports event-triggered transmissions,such as paging indications (e.g., when a paging message is received froma public land mobile network associated with the cell), RACH responsetransmissions (e.g., when a RACH is received from a mobile device campedor connected to the cell), or other idle-mode procedures for mobiledevices. In some implementations, such transmission can be supported bythe cell during the dormant or semi-dormant state in addition to theactive state. In other implementations, the base station can switch fromthe dormant state to the active state in response to such events andmake the related transmissions in the active state. In some otherimplementations, the base station can stay in dormant state for someevents and switch to active state for other events. For example, a basestation can transmit a paging indication while in the dormant state, andwait for a RACH transmission from the mobile device in response to thepaging indication, and after receiving the RACH transmission, switch tothe active state to transmit a RACH response to the mobile device.

In embodiments where the base station transmits a dormant stat messagesuch as on the D-PBCH, the dormant-state message includes one or moreparameters for the wireless network 160. Examples of the parametersinclude wake-up signal parameters, system information parameters,dormant-state parameters (e.g., paging parameters such as a subframeoffset for paging occasions for monitoring paging indications during thedormant state), active-state parameters, a PCID (e.g., forimplementations where the PCID cannot be implicitly determined fromSCID), a cell state indicator, a system frame number, or physical layerconfiguration parameters. Examples of wake-up signal parameters includea RACH preamble sequence index, an uplink evolved universal mobiletelecommunications system terrestrial radio access absolute radiofrequency number (“EARFCN”) of an uplink carrier for RACH transmissionby a mobile device, a subframe offset (e.g., the offset may be relativeto the subframe where the SCDS or D-PBCH is monitored by the mobiledevice) for RACH transmission, or a resource-block offset for RACHtransmission.

Examples of system information parameters include master informationblock parameters, a subframe offset (e.g., relative to the SCDS orD-PBCH subframe) to locate a master information block transmission whentransmitted during the active state, an update-indicator field, or avalue tag. The subframe offset information can be useful if the subframeindex used for SCDS or D-PBCH transmission is configurable (i.e., unlikePSS, SSS, or PBCH transmissions which occur in fixed andnon-configurable subframes (or at predetermined locations known to themobile device such as subframe 0 or 5 for frequency-divisionmultiplexing). The update-indicator field or value tag is a value (e.g.,an integer or flag) that indicates whether the system information haschanged. Examples of the cell state indicator include an active ordormant-state indicator or a carrier type indicator. In one example, thecell state indicator is an explicit indicator of the cell's state, suchas one bit that indicates a dormant or active state. In another example,the cell state indicator has two bits to indicate the dormant or activestate and also a carrier type being used by the cell in one or bothstates (e.g., legacy vs. new carrier type). Examples of the physicallayer configuration parameters include downlink and uplink bandwidth,antenna configuration, or power control parameters. The mobile devicecan receive such physical layer configuration parameters in MIB and SIBsbut transmitting them in the D-PBCH can potentially reduce the timerequired by the mobile device to transition from RRC_idle mode to“RRC_connected” mode, without waiting for the cell to transition toactive mode. While all of the above parameters can be signaled inD-PBCH, the additional flexibility provided by their transmission canresult in significantly increased overhead and reduced energy savingsduring dormant-state operation of the cell. Therefore, in someembodiments, the base station transmits only a subset of the parametersdescribed above and allows the mobile device to acquire other parametersfrom MIB or SIBs. For example, an implementation of D-PBCH (i.e., adormant-state message transmitted on the D-PBCH) can only include avalue tag that indicates whether the system information of the cell haschanged from a previous setting.

Turning to FIG. 3, a block diagram 300 depicts a possible implementationof the mobile device 150 of FIG. 1. The mobile device includes atransceiver 302 configured to transmit signals or data to and receivesignals or data from other devices such as the base station 110, 120,130, or 140 or other mobile device. The mobile device also includes aprocessor 304 that executes stored programs and at least one memory 306.The processor 304 writes data to and reads data from the memory 306. Themobile device includes a user interface 308 having a keypad, displayscreen, touch screen, microphone, speaker, or the like. Duringoperation, the transceiver 302 receives data from the processor 304 andtransmits RF signals representing the data. Similarly, the transceiver302 receives RF signals, converts the RF signals into appropriatelyformatted data, and provides the data to the processor 304. Theprocessor 304 retrieves instructions from the memory 306 and, based onthose instructions, provides outgoing data to, or receives incoming datafrom, the transceiver 302.

In an embodiment, the user interface 308 displays the output of variousapplication programs executed by the processor 304. The user interface308 additionally includes on-screen buttons that the user can press inorder to cause the mobile station to respond. The content shown on theuser interface 308 is generally provided to the user interface at thedirection of the processor 304. Similarly, information received throughthe user interface 308 is provided to the processor 304, which can thencause the mobile station to carry out a function whose effects may ormay not necessarily be apparent to a user.

The mobile device monitors for the SCDS from the base station tosynchronize with the wireless network 160. The mobile device scans anddetects the SCDS automatically (e.g., based on preconfiguredinstructions stored in the memory 306) or based on information orparameters received from another network element (e.g., another basestation) of the wireless network 160. The other network element in oneexample provides an indication of radio resources (e.g., carrierfrequencies, subframes, RBs, or resource elements) for the mobile deviceto use in scanning for the base station. In an idle mode, the mobiledevice monitors for a physical downlink control channel and pagingoccasions and, when camped on a dormant base station, the SCDS andSCDCH. In one example, the mobile device camps only on base stationsthat are in an active state.

Turning to FIG. 4, a flowchart depicts one example of a method for thebase station to provide an indication of a change in system information.The base station enters (405) an active state. During the active state,the base station transmits (410) system information using the sameprocedure as described in LTE Release 11 specifications. The systeminformation includes at least one SI message. The SI message can be aSIB1 message. The SIB1 message can be transmitted with a firstperiodicity (e.g., 20 ms or every alternate radio frame includingretransmissions). The SIB1 message includes a first update-indicatorfield. The first update-indicator field can be a first value tag field(e.g., an integer with range 0 to 31) that is associated with the systeminformation.

The base station determines (415) whether to enter a dormant state, forexample, based on a number of mobile devices currently connected,expiration of a timer (e.g., five minutes), or other factors. In oneexample, the base station enters the dormant state if it has notconfigured any mobile devices in an RRC_connected mode as described inLTE specifications for a predetermined time period (e.g., 5 minutes).The base station may set or modify the timer time period based on thetype of the mobile device that transmitted the wake-up signal during itsrecent transition from dormant to active state or the type of the mobiledevice it served recently in active state. For example, a high prioritymobile device such as a public safety device transmitting a particularwake-up signal (e.g., a RACH preamble from a reserved set of RACHpreambles for high priority mobile devices) may result in the basestation timer time period value to be set to a different value from thatof receiving a wake-up signal from a normal priority mobile device. Ifthe base station does not enter the dormant state (NO at 415), then thebase station continues transmitting (410) system information with thefirst periodicity. While in the active state, the base station selects(420) an update value (e.g., an integer) for the first update-indicatorfield. The first update-indicator field provides an indication to amobile device as to whether the system information has changed since aprevious transmission of a previous SIB1 message. Thus, if the systeminformation has not changed, then the base station continues to transmit(410) the SI messages using the same update value for the firstupdate-indicator field.

During the dormant state (YES at 415), the base station transmits (425)a dormant-state message, as described above, at the second periodicity(e.g., 100 ms, 1 second, or more) that is greater than the firstperiodicity. The dormant-state message may be transmitted on a D-PBCH.The dormant-state message includes a second update-indicator field thatprovides an indication to a mobile device as to whether the systeminformation has changed. The second update-indicator field can be asecond value tag field. The second update-indicator filed can includethe same update value as the first update-indicator field of thepreviously sent SIB1 message in the active state if the systeminformation has not changed. In some embodiments, when the base station(or the cell) transitions to a dormant state, the mobile devices thatare camped on the cell, but not connected to that cell, can reselect toa different cell or initiate a wake up procedure to transition the cellfrom dormant to active state.

The base station determines (430) whether a wake-up request has beenreceived. Examples of wake-up requests include wake-up requests frommobile devices camped on the base station, wake-up requests from othernetwork elements of the wireless network 160, or wake-up requestsgenerated within the base station. The base station enters (405) theactive state if the wake-up signal is received (YES at 430). If thewake-up request has not been received (NO at 430), then the base stationdetermines (435) whether the system information has changed. If thesystem information has not changed (NO at 435), then the base stationcontinues to transmit (425) dormant-state messages with the update valuefrom the update-indicator field of the previously sent SI message as thevalue for the second update-indicator field. If the system informationhas changed (YES at 435), then the base station returns to the activestate (405) and sends (410) the updated system information. In oneembodiment, the cells support, in the dormant state, some or all of theprocedures required to service idle mobile devices. In oneimplementation of this embodiment, if the cell is in dormant state, thenthe mobile device can determine some information from the SCDS and abroadcast channel that is transmitted along with the SCDS. Theinformation may include detailed parameters described above such as (a)whether the system information transmitted by the cell has changed froma previous setting (e.g., using a value tag transmitted in D-PBCH), (b)wake-up signal parameters that are optionally transmitted (e.g., a RACHpreamble sequence index, ULplink EARFCN of an uplink carrier where themobile device can transmit a RACH, subframe offset for RACHtransmission, RB offset for RACH transmission), and (c) dormant-statepaging parameters that are optionally transmitted (e.g., a subframeoffset for monitoring paging indication in dormant mode).

Turning to FIG. 5, a flowchart illustrates one example of a method forthe mobile device to determine whether the base station is in an activestate or dormant state based on the SCDS. The mobile device receives(505) an SCDS from the base station. The mobile device performs (510) adetection attempt on the received signal. The detection attempt is basedon a characteristic of the received signal such that if the detectionattempt is successful, then the mobile device determines that the basestation is in either the active state or the dormant state. If thedetection attempt is successful (YES at 515), then the mobile devicedetermines (520) a current state of the base station (e.g., the activestate, dormant state, or semi-dormant state). If the detection attemptis not successful (NO at 515), then the mobile device performs (510) atleast one second detection attempt. The mobile device performs aseparate detection attempt that is distinct from the previous detectionattempt. For example, the mobile device performs a first detectionattempt that corresponds to an active state for the base station and asecond detection attempt that corresponds to a dormant state for thebase station. In some embodiments, the mobile device performs aplurality of detection attempts, in a sequence or in parallel, to detectthe received signal.

The base station selects and transmits the SCDS based on its currentstate, thus allowing the mobile device to determine the current statebased on the received signal. In one embodiment, the base stationselects a cell identifier (e.g., an SCID) from a first set of cellidentifiers that correspond to an active state or from a second set ofcell identifiers that correspond to a dormant state. The first andsecond sets of cell identifiers can be disjoint. The base stationtransmits a pilot sequence for the SCDS based on the selected cellidentifier. The mobile device performs (510) one or more detectionattempts blindly to detect the transmitted pilot sequence and thusdetermine the corresponding cell identifier. In this case, the mobiledevice determines that the base station is in the active state if thedetected cell identifier is in the first set of cell identifiers and inthe dormant state if the detected cell identifier is in the second setof cell identifiers. In one example, the second set of cell identifiersis distinct from the first set of cell identifiers.

In another embodiment, the second set of cell identifiers is offset fromthe first set of cell identifiers by a predetermined value. For example,the base station generates the pilot sequence based on a first SCID X ifthe base station is in the active state, and, if in the dormant state,then the pilot sequence is based on X+N where Xis the SCID of the celland N is the predetermined value. The predetermined value may be a largeinteger greater than the largest allowed SCID value. If the possible setof SCID values is {0, 1, . . . , 4031}, then the predetermined valueN=4032. In this case, for each SCID value X, the mobile device makes twoattempts for detection: a first attempt based on a pilot sequencegenerated using X and a second attempt based on a pilot sequencegenerated using X+N. If the first detection attempt succeeds, then themobile device determines that the base station is in the active state.If the second detection attempt succeeds, then the mobile devicedetermines that the base station is in the dormant state.

According to another embodiment, the base station generates the pilotsequence for the SCDS using a Gold sequence generator (similar to theone described in 3rd Generation Partnership Project TechnicalSpecification 36.211) or another well known sequence generator. In thiscase, the base station initializes the Gold sequence generator with afirst initialization state during the active state and with a secondinitialization state during the dormant state. The initialization statesin one example are functions of a state bit that corresponds to theactive state or dormant state (e.g., 0 for dormant, 1 for active). Inthis case, the mobile device performs a first detection attempt assumingthat the SCDS is generated using a gold sequence that is initializedusing a first value and a second attempt assuming that the SCDS isgenerated using a gold sequence that is initialized using a secondvalue. The mobile device determines (510) whether the transmitted pilotsequence corresponds to a first gold sequence generated with the firstinitialization state or a second gold sequence generated with the secondinitialization state. If the first detection attempt succeeds, then themobile device determines that the base station is in the active state.If the second detection attempt succeeds, then the mobile devicedetermines that the base station is in the dormant state.

In another example, the base station uses a same base sequence (e.g.,one generated based on the SCID) for the SCDS and selects a scramblingsequence based on the current base station state. The scramblingsequence is applied to the base sequence to generate the pilot sequencefor the SCDS. In another example, the base station uses the same basesequence (e.g., generated based on the SCID) for the SCDS and selects acyclic shifts value based on the current base station state. In yetanother example, the base station transmits the SCDS using a first setof radio resources (e.g., time-frequency resources) during the activestate and using a second set of radio resources during the dormantstate. In another example, the base station uses different resourcehopping patterns for the SCDS time-frequency resource/resource mappingas a function of the base station state. In yet another example, thebase station uses different pilot sequence hopping patterns for the SCDSas a function of the base station state.

The mobile device in one embodiment performs one or more detectionattempts for active base stations before performing detection attemptsfor dormant base stations. In this case, the mobile station only scansfor dormant base stations if no active base stations are detected. Inanother example, the mobile device uses assistance information fromanother network entity that indicates one or more SCIDs with a higherpriority. In this case, the mobile device performs detection attemptsfor the indicated SCIDs in both the active and dormant state beforeproceeding to other SCIDs.

The mobile device can use different radio resources, such as a pagingoccasion, based on the current state of the base station. If the basestation is in the active state (YES at 520), then the mobile device uses(525) a first set of radio resources (e.g., active-state pagingoccasions). If the base station is in the dormant state, then the mobiledevice uses (530) a second set of radio resources (e.g., dormant-statepaging occasions). As one example, the mobile device scans for a pagingmessage from the base station using a first set of paging radioresources if the base station is in the dormant state and using a secondset of paging radio resources if the base station is in the activestate.

The first set of radio resources for the active state is different fromthe second set of radio resources for the dormant state. In one example,the second set is selected such that the random access channel or pagingoccasions for the mobile devices are in RBs or subframes that areadjacent or near the SCDS. This allows the mobile device to scan for apaging indication, respond to the paging indication, and maintainsynchronization with the wireless network 160 via the SCDS by scanningonly a relatively small period of time. This technique reduces the powerconsumption of the mobile device and the base station by reducing theperiod of time over which they must scan or transmit.

In one embodiment, the first and second sets of radio resources arepredetermined by a network operator of the wireless network 160. In thiscase, the mobile device and base station store an indication of thefirst and second sets of radio resources. In another embodiment, thebase station indicates the second set of resources by providing one ormore dormant-state parameters in the system information provided duringthe active state. Examples of the dormant-state parameters include asubframe offset or explicit RBs that are selected by the base station.

The above-described mechanisms allow the mobile station to differentiatebetween active and dormant base stations in cell discovery and searchprocedures. For example, a mobile device can perform a cell discoveryand search method where it first scans for all SCDS sequences thatcorrespond to active base stations, and only if it is unable to detectan active base station, it then proceeds to scan for SCDS sequencescorresponding to dormant base stations. In another example, the mobiledevice can scan for some preferred SCIDs corresponding to some preferredbase stations (e.g., via assistance information) using both active stateSCDS sequences and dormant-state SCDS sequences, and if the mobiledevice does not detect its preferred base stations in either active ordormant state, it can then proceed to scan for other base stations usingtheir active state SCDS sequences first and later using theirdormant-state SCDS sequences. These approaches help in reducing theoverall time taken by the mobile devices to not only detect a basestation (which can be done when the base station is in the active ordormant state) but also to transmit and receive user-specific datatraffic to or from the base station (usually this is done when the basestation is in active state).

A longer periodicity broadcast channel transmitted along with SCDS(e.g., D-PBCH) can also be used for indicating the active or dormantstate of a base station. This approach reduces a worst-case SCDSscanning burden on the mobile device as the mobile device has to onlysearch for one set of SCDS sequences irrespective of base station state(instead of multiple sets based on base station state as describedearlier). However, this approach can increase an average scanning burdenas the mobile device cannot preemptively ignore dormant base stations inits scanning procedure as it first has to successfully detect the SCDSof a base station before proceeding to decode the broadcast channelwhere the indication of base station state is present.

Turning to FIG. 6, a flowchart illustrates one example of a method forthe mobile device to send a wake-up request to a base station in adormant state. The mobile device wakes the base station to transition toan active state in order to camp on the base station, receive systeminformation, perform a handover, or send a data transmission. In thiscase, the mobile device camps only on cells that are in an active state.The mobile device receives (605) a signal from the base station. In afirst example, the signal is the SCDS. In another example, the signal isthe dormant-state message transmitted via the dormant physical broadcastchannel.

The mobile device determines (610) whether the base station is in thedormant state or the active state based on the received signal (asdescribed herein). If the base station is in the active state (NO at610), then the mobile device monitors (615) the base station using thefirst set of radio resources, as described above with reference to FIG.5. If the base station is in the dormant state (YES at 610), then themobile device sends (620) a wake-up request to the base station. Inother embodiments, the mobile device sends the wake-up request to a basestation if the mobile device has not yet received system informationfrom the base station. The wake-up request is a signal, message, orpreamble which triggers the base station to transition to the activestate.

The base station transmits wake-up signal parameters to the mobilestation, which the mobile station then uses to transmit the wake-upsignal. The base station monitors for the wake-up request using theradio resources indicated to the mobile device. As described above,examples of wake-up signal parameters include a RACH preamble sequenceindex, an uplink EARFCN of an uplink carrier for RACH transmission bythe mobile device, a subframe offset for RACH transmission, or aresource-block offset for RACH transmission. In one example, the mobiledevice transmits a RACH preamble sequence as the wake-up signal (e.g.,with one of the pre-specified formats described in LTE Release 8, 9, 10,or 11) in a subframe and a set of RBs on an uplink carrier. The RACHpreamble sequence can be selected from a set of possible RACH preamblesequences based on the detected SCID or type of mobile device. Themobile device can determine the set of RBs and the EARFCN of the uplinkcarrier from the SCID, SCDS, or Dormant-PBCH. The subframe in which themobile device transmits the RACH can have a fixed timing offset from thesubframe in which it detects SCDS or can be based on the SCID.Alternately, if a Dormant-PBCH message is received by the mobile device,then a subframe offset can be included in that message. For example, ifthe mobile device detects SCDS in subframe p, then it can transmit theRACH in subframe p+W where W is the subframe offset.

After sending the wake-up request, the mobile device continues tomonitor the SCDS or D-PBCH to determine (625) the current status of thebase station. For example, the mobile device determines whether the basestation has transitioned from dormant to active state based on a changein SCDS, SCID, or Dormant P-BCH. If the base station has transitioned tothe active state (YES at 630), then the mobile device receives (615) thesystem information from the base station. In this case, the mobiledevice downloads system information from the base station and startsmonitoring the PDCCH or EPDCCH of the base station for pagingindications.

While the base station has not transitioned to the active state (NO at630), the mobile device optionally starts a wake-up timer after sendingthe wake-up request. If the wake-up timer has not expired (NO at 635),then the mobile device continues to determine (625) the current statusof the base station. If the wake-up timer has expired (YES at 635), thenthe mobile device sends another wake-up request to the base station. Inother embodiments, if the mobile device is unable to detect thetransition to the active state after several transmissions of thewake-up signal, then the mobile device selects another base station onwhich to camp.

In some embodiments, the mobile device camps only upon active cells. Inthis case, information that a cell has to support in the dormant stateis minimized. In some cases, it is possible to not transmit a longerperiodicity broadcast channel along with SCDS thereby saving time,frequency, and energy resources. For example, a cell can only supportSCDS transmissions (e.g., a pilot sequence spanning mapped to multiplesymbols and a subset of subcarriers in a subframe) in the dormant stateand wake-up parameters (e.g., information on how to wake up the cell)can be determined by the mobile device from the SCID detected from theSCDS and a set of predetermined rules (e.g., a predefined subframeoffset from the subframe on which SCDS is received or a predefinedmapping table that indicates a preamble sequence to use when aparticular SCID is detected). In dense small cell networks, many smallcells may not have mobile devices camped in idle mode within theircoverage area. Since paging indications (e.g., a PDCCH with downlinkcontrol information scrambled by a paging radio network temporaryidentity) or messages (e.g., a paging medium access control elementtransmitted on PDSCH) are only required to be transmitted by cells thathave been switched to active state by idle-mode mobile devices that arecamped on them, unnecessary paging transmissions in cells that do nothave any idle-mode mobile devices are reduced.

Where the mobile device only camps on active base stations, proceduresthat are required to service idle-mode mobile devices (i.e.,transmission of system information and paging messages) may not besupported in the dormant state. In scenarios where most small cells haveidle-mode mobile devices camped within their coverage area, the benefitsor supporting a dormant state are not fully realized as most cells areforced to be in active state. Also, if a mobile device is camped on acell for a prolonged period without establishing a connection to thecell (i.e., the mobile device has no data incoming from or outgoing tothe cell), then the cell may time out and move to dormant state, and themobile device may have to repeatedly wake up the cell (e.g., once every5 minutes) to move it back to active state.

Turning to FIG. 7, a flowchart illustrates a method for determining acell identifier, such as a PCID, based on a SCDS. In this case, the basestation determines a SCDS (e.g., a pilot sequence) based on a PCID, suchas, for example, using the PCID as an initialization value or an offsetvalue in the construction of the pilot sequence used for SCDStransmission. In another example, the base station uses the PCID as aninitialization or offset value for selection of radio resources orresource mapping used for transmission of the SCDS. In another example,one or more characteristics of the SCDS are based on at least a portionof the PCID value. The characteristic of the SCDS can include pilotsequence waveform, cyclic shift value of the SCDS pilot sequence, pilotsequence hopping pattern on different time-frequency resources (e.g.,different pilot sequence waveforms on different OFDM symbols), resourcehopping pattern of the SCDS time-frequency resource/resource mapping(e.g., different set of subcarriers on different OFDM symbols), etc.This allows the mobile device to attempt to detect the SCDS byhypothesizing various values of the PCID from a set of allowed orconfigured values. The hypothesized value that meets a predetermineddetection threshold of the SCDS is then determined to be the PCID forthat base station. As one example, the set of allowed values includesthe values of {0, 1, . . . , 503} as defined in LTE Releases 8, 9, 10,and 11.

The base station in another example uses an SCID for identifying thesmall cell. In one example, the base station is associated with both anSCID and also a PCID. The base station selects the SCID from a set ofallowable values that is larger than the set of allowable values for thePCID. As one example, the set of allowable values for the SCID is {0, 1,. . . , 4031}. As another example, the set of allowable values for theSCID is {504, 505, . . . , 4031}. In this case, the SCID does notoverlap with PCIDs to reduce confusion in measurement reports. Themobile device determines the SCID by hypothesizing various values of theSCID from the larger set of allowable values for the SCID and selectsthe SCID that meets a predetermined detection threshold.

Where the base station uses the SCID, the base station in one exampledetermines other pilot signals, reference signals, or control channelsbased on the SCID instead of the PCID. In this case, the base stationoptionally does not support communications based on LTE Releases 8, 9,10, and 11 but instead supports a standalone new carrier type. Forexample, a mobile device connected to a base station associated with thestandalone carrier uses the SCID to determine a sequence used for asynchronization or tracking reference signal or channel stateinformation reference signal from that base station. The mobile devicecan also use the SCID to receive control channels such as an enhancedphysical downlink control channel from the base station.

In another example, the base station uses both the SCID as well as thePCID. In this case, the base station transmits pilot and controlchannels based on the PCID (e.g., the cell reference signals andphysical downlink control channel). The base station selects the SCIDbased on the PCID so that the PCID is determinable based on the SCID.This technique allows the mobile device to use a single cell identifierdetection procedure for determination of the SCID (based on the SCDS)and thereby determine the PCID, instead of using the PSS and SSS todetermine the PCID.

The base station determines (705) a physical-cell identifier (i.e., thePCID). The PCID in one example is provisioned by a network operator ofthe wireless network 160. The base station selects (710) the SCID basedon the PCID. The base station may use a many-to-one mapping between theSCID and the PCID. As one example, the base station selects the SCIDsuch that the mobile device can determine:

-   -   PCID=Modulo (SCID, 504).

In this case, SCIDs 0, 504, and 1008 map to PCID 0, while SCIDs 1, 505,and 1009 map to PCID 1, and SCIDs 503, 1007, and 1511 map to PCID 503.The base station broadcasts (715) the SCDS based on the SCID.

Turning to FIG. 8, a flowchart depicts one example of a method for themobile device to determine the PCID of a cell based on a received SCID.The mobile device receives (805) an identification signal from the basestation, such as the SCDS. The mobile device selects (810) a firstidentifier value from a first set of available identifiers, such as theset of available SCIDs. As described above, the mobile device determinesthe small-cell identifier (the first identifier) by hypothesizingvarious values of the SCID from the larger set of allowable values forthe SCID and selects the SCID that meets a predetermined detectionthreshold. The mobile device maps (815) the first identifier to a set ofavailable second identifiers, such as the set of available PCIDs using amany to one mapping. For example, the mobile device can map the firstidentifier as to the second identifier (PCID) as:

-   -   PCID=Modulo (SCID, 504) where SCID is first identifier and PCID        is second identifier. Thus, the mobile device determines the        second identifier based on the first identifier.

The mobile device then receives (820) communications, such as thecontrol channels, from the base station using the determined PCID orsecond identifier.

Turning to FIG. 9, a flowchart illustrates an example of a method forthe mobile device to determine the SCID of a cell based on a receivedsignal. This implementation can include PRS sequence initialization andv_(shift) using the SCID instead of PCID. In LTE Release 11, the mobiledevice searches for 504 PCIDs in the absence of a neighbor cell list. Onthe other hand, providing a neighbor cell list can reduce the mobiledevice searching burden, but this requires significant operation andmaintenance effort and the implementation of self-optimizing network andautomatic neighbor relations functions. With the SCDS and SCIDsdescribed herein, there can be more than 504 SCIDs, thus methods toreduce the mobile device search space can be beneficial. The basestation and mobile device in one example use SCIDs that are organizedinto groups. This technique reduces search burden on the mobile devicewhen hypothesizing the SCID. In one example, a total number of sequenceidentifiers, i.e., SCIDs, for SCDSs is an integer multiple of the numberof available PCIDs (i.e., 504 K where K is an integer).

-   -   The network operator defines a small cell group identifier        (“SGID”) as: SGID=Modulo (SCID, G).

In this case, G is of the form:G=2^(j)3^(k)7^(l) where j=0,1, . . . ,6; k=0,1,2,3 and l=0,1.

The set of SCIDs are divided into G distinct groups with each of thedistinct groups indexed by the SGID. For a number M=4032 of SCIDs and anumber G=504 (e.g., where j=3, k=2, l=1), an SGID=0 corresponds to SCIDs{0, 504, 1008, 1512, 2016, 2520, 3024, 3528} while SGID=1 corresponds toSCIDs {1, 505, 1009, 1513, 2017, 2521, 3025, 3529). In this case, eachgroup identified by an SGID contains (M/G) elements. For M=4032, thereare M/G=8 SCIDs within each SGID group. The base station scramblesnon-legacy signals, such as an enhanced physical downlink controlchannel, physical downlink shared channel, enhanced cell referencesignal, or enhanced channel state information reference signals, basedon a sequence that is a function of the SGID.

This results in the mobile device having to search for G=504 SGID groupsinstead of M=4032 SCIDs, which allows for a similar search complexity asin LTE Release 8. Scrambling and decoding of the non-legacy signals withthe SGID by the base station and mobile device, respectively, areanalogous to the scrambling and decoding of signals with the PCID, aswill be apparent to those skilled in the art.

The mobile device receives (905) a non-legacy signal as a first signal.The first signal in one example is the SCDS. The mobile devicedetermines (910) the group identifier SGID based on the first signal,for example, by performing one or more detection attempts as describedabove with reference to FIG. 5. Once the SGID is determined by themobile device, the mobile device resolves the SCID to an element withinthe group of 8 SCIDs that corresponds to the detected SGID. The mobiledevice receives (915) a second signal from the base station. The mobiledevice selects (920) the set of SCIDs within the group of SCIDs thatcorrespond to the detected SGID. The mobile device detects (925) thesecond signal based on the selected set of SCIDs and determines (930)the SCID based on the decoding attempt.

The mobile device in one example determines the SCID based on a randomaccess channel response as the second signal from the small cell. Inthis case, the mobile device blindly descrambles the physical downlinkcontrol channel using hypothetical SCIDs that correspond to the detectedSGID. In another example, the mobile device descrambles a dormantphysical broadcast channel as the second signal using the hypotheticalSCIDs that correspond to the detected SGID. In yet another example, themobile device performs blind detection of pilot signals as the secondsignal, such as enhanced cell reference signals or enhanced channelstate information reference signals, from the base station. In thiscase, the pilot signals are scrambled or initialized by a function ofthe SGID, in contrast to LTE Release 11, where the signals are scrambledor initialized by a function of PCID. If SCDS assistance information isprovided by a serving cell (e.g., via the system information block,radio resource control information, etc.) to the mobile device, then thetransmission bandwidth of the SCDS can be conveyed via the assistanceinformation.

If the mobile device is expected to detect the SCDS without relying onassistance information signaling from another cell, then the mobiledevice can be required to perform blind decoding of SCDS. In such acase, to reduce blind decoding complexity, some simplifications may bepossible, such as using a transmission bandwidth for SCDS of 1.4 MHz fora system bandwidth <=5 MHz and a transmission bandwidth of 5 MHz forSCDS for a system bandwidth >5 MHz. Alternately, if SCDS assistanceinformation is provided by a serving cell (e.g., via SIB, radio resourcecontrol, etc.) to the mobile device, then the transmission bandwidth ofthe SCDS can be conveyed via the assistance information.

Turning to FIG. 10, a flowchart illustrates an example of a method forthe mobile device to determine a system bandwidth for the base station.The base station in one example uses a bandwidth selected from a set ofpredetermined bandwidths (e.g., 1.4 MHz and 5 MHz) for transmission ofthe SCDS. The mobile device thus looks for both 1.4 MHz and 5 MHz SCDSsequences blindly, so for M=4032, the search space is 2*4032=8064. Thebase station can transmit a 5 MHz small signal discovery signal in twoconsecutive subframes once every T₁=200 milliseconds. The base stationcan transmit a 1.4 MHz SCDS in six consecutive subframes once everyT₁=200 milliseconds. A sequence (for quadrature phase shift keying) andv_(shift) can use the same mapping based on PCID as is the case for PRS.The frequency re-use and the length of the sequence (5 MHz+2milliseconds or 1.4 MHz+6 milliseconds) is expected to provide superiordetectability as compared to PSS and SSS or even cell reference signals,enabling the network operator to configure a large number of small cellswithin one or two carrier frequencies f₁ and f₂.

The mobile device scans for both the 1.4 MHz and 5 MHz SCDSs (e.g., theset of predetermined bandwidths) simultaneously (e.g., “one shot” celldetection). The mobile device receives (1005) a first signal from thebase station during a predetermined buffer period that corresponds tothe length of the sequence (e.g., 6 ms). The predetermined buffer periodcan be shorter if only the 5 MHz SCDS is present. The mobile deviceperforms (1010) a first decoding attempt on the first signal using afirst bandwidth (e.g., 1.4 MHz) from the set of predetermined bandwidthsand also performs (1015) a second decoding attempt on the first signalusing a second bandwidth (e.g., 5 MHz) from the set of predeterminedbandwidths. The mobile device determines (1020) which bandwidth is usedfor the SCDS, the 1.4 MHz or 5 MHz, based on whether the first or seconddecoding attempt is successful.

In one example, all small cells in a dormant state transmit the SCDS onjust one or two carrier frequencies, such as the f₁ or f₂ frequencies inthe 3.5 GHz band. The transmit occasions on each carrier aresynchronized (e.g., using over the air network listening). The SCDStransmit occasions on carrier f₁ are on SFN index S1 and subframe n1,such that S1=Modulo(SFN, T1). The SCDS transmit occasions on carrier f₂are on SFN index S2 and subframe n2, such that S2=Modulo (SFN, T2). Inone example, T1=T2=200 milliseconds such that a mobile device served bya macro cell only needs to look at two carriers to detect dormant smallcells. The macro cell can broadcast or indicate in a dedicated messagethe SCDS transmit schedule (e.g., offset (S1/S2) and periodicity(T1/T2)). In other examples, the macro cell schedules inter-frequencymeasurement gaps aligned with the transmit occasions if there is justone layer f₁.

In another embodiment, a mobile device (e.g., the mobile device 150)receives a signal from a cell (e.g., from the cell 131 via the basestation 130). The signal can be the SCDS. The mobile device determines,based on the received signal, a first identifier value (e.g., a cellidentifier or small-cell identifier value) belonging to a first set ofvalues, such as the set of possible SCID values e.g., {0, 1, . . . ,4031}. The mobile device determines, from the first identifier value, asecond identifier value, such as a physical-cell identifier or PCIDvalue. The second identifier value belongs to a second set of values,such as a set of possible PCID values, i.e., {0, 1, . . . , 503}. Thefirst set is larger than the second set. The mobile device determinesthe second identifier value based on a many to one mapping between thefirst set and the second set. The mobile device receives one or morephysical channels or signals (e.g., PBCH, PCFICH, PDCCH, or CRS) fromthe cell using the second identifier value. The mobile device canreceive the discovery signal when the cell is in a dormant state. Themobile device can receive the one or more physical channels when thecell is in an active state. The mobile device can determine the secondidentifier value from the first identifier value using the followingmany to one mapping rule: second identifier value=MOD (first identifiervalue, 504).

According to another embodiment, a mobile device performs the steps of:receiving a discovery signal (e.g., a cell-discovery signal) from acell, attempting to detect a pilot sequence associated with the receivedsignal by making a first hypothesis and at least a second hypothesis,determining that the cell is in active state if the attempt issuccessful by making the first hypothesis, and determining that the cellis in dormant state if the attempt is successful by making the secondhypothesis.

The mobile device can also perform the steps of: assuming that the pilotsequence is generated using a cell identifier chosen from a first set ofvalues, and assuming that the pilot sequence is generated using a cellidentifier chosen from a second set of values.

The step of making the first hypothesis can further include: assumingthat the pilot sequence is generated using a cell identifier chosen froma first set of values. The step of making the second hypothesis canfurther include: assuming that the pilot sequence is generated using anoffset value added to the cell identifier chosen while making the firsthypothesis.

The step of making the first hypothesis can further include assumingthat the pilot sequence is generated using a first initialization stateof a gold sequence generator. The step of making the second hypothesiscan further include assuming that the pilot sequence is generated usinga second initialization state of a gold sequence generator.

The first set can be distinct from the second set.

According to yet another embodiment, a mobile device performs the stepsof: receiving a signal, such as a cell-discovery signal, decoding thereceived signal to detect a pilot sequence associated with an identifierof a cell, the pilot sequence corresponding to transmission of adiscovery signal of the cell, determining the received signal containsthe discovery signal of the cell based on the decoding attempt, andacquiring the energy saving state of the cell (e.g., a dormant or activestate) based on the result of the detecting.

In another embodiment, a mobile device determines a cell groupidentifier and then determines a cell sequence identifier, such as asmall cell sequence identifier (“SSID”). The mobile device performs thesteps of: receiving a cell-discovery signal from a base station,receiving a second signal different from the cell-discovery signal,determining a group identifier of the base station from thecell-discovery signal, decoding the second signal based on a hypothesison a SSID of the base station where the hypothesis is drawn from a setdetermined from the group identifier of the base station, anddetermining the SSID of the base station. The second signal can be oneof a RACH response signal, a D-PBCH signal, a cell-specific referencesignal scrambled or initialized with the SSID of the base station, or achannel state information reference signal scrambled or initialized withthe SSID of the base station.

According to another embodiment, a mobile device performs “blind”detection of a bandwidth for a cell-discovery signal. The mobile devicereceives a cell-discovery signal on a first carrier frequency, whichincludes hypothesizing that the cell-discovery signal has at least oneof a first bandwidth and a second bandwidth. The mobile devicedetermines the bandwidth of the cell-discovery signal based on thereceived cell-discovery signal.

In another embodiment, a base station transmits messages in an activestate and a dormant state. The base station transmits SI messages for aPublic Land Mobile Network (“PLMN”) in a plurality of SIBs whenoperating in an active state. The plurality of SIBs includes a SIB1message. The base station periodically transmits the SIB1 with a firstperiodicity. The SIB1 includes a first value tag field that indicates ifa change has occurred in SI messages for the PLMN. The first value tagfield has a first value. The base station transmits a dormant-statemessage when operating in a dormant state. The base station periodicallytransmits the dormant-state message with a second periodicity that islonger than the first periodicity. The dormant-state message includes asecond value tag field. The second value tag field has the first valueif SI messages for the PLMN are unchanged. The second value tag fieldhas a second value if SI messages for the PLMN are changed.

According to yet another embodiment, a mobile device performs the stepsof: determining if a cell is operating in an active state or a dormantstate, receiving paging messages from the cell in a first set of timedomain occasions when the cell is operating in active state, receiving adiscovery signal in a second set of time domain occasions when the cellis operating in dormant state, and receiving paging messages from thecell in a third set of time domain occasions when the cell is operatingin dormant state. The mobile device determines the third set of timedomain occasions from the second set of time domain occasions. At leastone time domain occasion in the third set of time domain occasions isdifferent from at least one time domain occasion in the first set oftime domain occasions. The third set of time domain occasions and thesecond set of time domain occasions may occur in adjacent subframes.Alternate paging locations such as the third set of time domainresources can be next to a discovery signal location in order to helpreduce power consumption by the mobile device, since it can receive boththe paging message and discovery signal in one wake up interval. Inother embodiments, the mobile device can transmit RACH preambles inalternate time domain locations when the cell is in the dormant state,analogous to receiving the paging messages in alternate time domainlocations.

In another embodiment, a base station operates in at least one of anactive state and a dormant state. The base station performs the stepsof: transmitting a discovery signal associated with the base stationidentifier when operating in a dormant state and transmitting a firstmessage when operating in the dormant state. The first message includesat least a value tag field associated with system-information messagesand a field indicating that the base station is in a dormant state. Thevalue tag field indicates a first value.

The base station can further perform the steps of: acquiring updatedsystem-information messages, transmitting a second dormant-statemessage, the second message including at least a value tag field and afield indicating the base station is in a dormant state, the value tagfield indicating a second value that is not same as the first value, andtransmitting at least the updated system-information messages during thedormant state.

The base station can further perform the steps of: acquiring informationto transition from the dormant state to active state and transmittingsystem-information messages during the active state. Thesystem-information messages can include at least one message including avalue tag field associated with the system-information messages. Thevalue tag field can indicate the first value if the system-informationmessages are unchanged when transitioning from the dormant state toactive state. The value tag field can indicate a second value if thesystem-information messages are changed when transitioning from thedormant state to active state.

The disclosed embodiments may be described in terms of functional blockcomponents and various processing steps. Such functional blocks may berealized by any number of hardware or software components configured toperform the specified functions.

It can be seen from the foregoing that methods and devices forcommunication between mobile devices and base stations with active anddormant states are provided. In view of the many possible embodiments towhich the principles of the present discussion may be applied, it shouldbe recognized that the embodiments described herein with respect to thedrawing figures are meant to be illustrative only and should not betaken as limiting the scope of the claims. Therefore, the techniques asdescribed herein contemplate all such embodiments as may come within thescope of the following claims and equivalents thereof.

We claim:
 1. A method in a base station for a long term evolutionnetwork, the method comprising: transmitting, during an active state ofthe base station, system information with at least onesystem-information message, wherein the at least one system-informationmessage comprises a SystemInformationBlockType1 (“SIB1”) message with afirst update-indicator field; selecting an update value that indicateswhether the system information has changed since a previous transmissionof a previous SIB1 message; and transmitting, during a dormant state ofthe base station, at least one dormant-state message with the selectedupdate value in a second update-indicator field of the at least onedormant-state message.
 2. The method of claim 1 wherein selecting theupdate value comprises: selecting a previous value of the firstupdate-indicator field of the previous SIB1 message if the systeminformation has not changed since the previous transmission; andselecting a value different from the previous value if the systeminformation has changed since the previous transmission.
 3. The methodof claim 2 further comprising returning to the active state of the basestation to transmit updated system information if the system informationhas changed since the previous transmission.
 4. The method of claim 1:wherein transmitting at least one system-information message comprisestransmitting a plurality of system-information messages with a firstperiodicity; and wherein transmitting at least one dormant-state messagecomprises transmitting a plurality of dormant-state messages with asecond periodicity that is greater than the first periodicity.
 5. Themethod of claim 1 wherein transmitting at least one dormant-statemessage comprises transmitting at least one dormant-state message in adormant physical broadcast channel.
 6. The method of claim 5 furthercomprising transmitting only the dormant physical broadcast channel anda cell-discovery signal associated with an identifier of the basestation during the dormant state.
 7. The method of claim 6 furthercomprising transmitting radio-resource information to be used for uplinkcommunication with the base station during the dormant state that isdistinct from radio-resource information to be used for uplinkcommunication with the base station during the active state.
 8. Themethod of claim 7: wherein the radio-resource information indicatesradio resources to be used for a wake-up request for the base station;the method further comprising scanning the indicated radio resources forthe wake-up request.
 9. The method of claim 8: wherein theradio-resource information indicates a wake-up preamble sequence to beused for the wake-up request; and wherein scanning the indicated radioresources comprises scanning the indicated radio resources for thewake-up preamble sequence.
 10. The method of claim 6 further comprisingtransmitting radio-resource information including dormant state pagingoccasions that are distinct from active state paging occasions.
 11. Themethod of claim 10 further comprising paging a mobile device during thedormant state using the dormant state paging occasions.